1. Field of the Invention
The present invention relates to a column driving method for liquid crystal display devices and, more particularly, a hybrid-level charge recycling method for use in implementing low power LCD column drivers.
2. Description of the Prior Art
With increasing development and use of mobile applications like the digital notebook, the design technique of a low-power LCD column driving integrated circuit (IC) becomes more important. While image quality, EMI reduction, packaging dimensions and weight are also important issues in LCD driving devices, of particular importance are power dissipation and the number of external components needed.
FIG. 1 shows a block diagram of a conventional column driver.
The conventional column driver comprises a digital logic unit 10, a digital-analog converter (DAC) 12, an output driving unit 14, and a charge recycling unit 16 in order to drive the an LCD panel 18.
Digital image information, such as red (R), green (G) and blue (B) data, is inputted to the digital logic unit 10, in order to drive the DAC 12. An analog voltage generated by the DAC 12 is inputted to the output driving unit 14. At this time, the output driving unit 14 drives column lines having relatively large capacitance.
The power consumption of the output driving unit 14 for driving the column lines is highest among the elements comprised in the column driver. The dynamic power consumption can be expressed as equation (1).                               P          dynamic                =                                            V              dd                        xc3x97                          I              avg                                =                                                    V                dd                            xc3x97              N              xc3x97                              C                load                            xc3x97                              V                swing                            xc3x97                              F                row                                      2                                              (        1        )            
In equation 1, the xe2x80x98Vddxe2x80x99 is power supply voltage, xe2x80x98Iavgxe2x80x99 is average current from the power supplier, xe2x80x98Nxe2x80x99 is the number of outputs in the driver, xe2x80x98Cloadxe2x80x99 is capacitance of the respective column line, xe2x80x98Frowxe2x80x99 is row frequency or horizontal frequency, and xe2x80x98Vswingxe2x80x99 is the average output voltage swing.
As shown in FIG. 1, the column driver comprises the charge recycling unit 16 for charge conservation in order to reduce the power consumption. The charge conservation method employed in the charge recycling unit 16 will be described referring to FIGS. 2A and 2B.
FIG. 2A shows the circuit of the conventional column driver adopting a 1-level/3-phase charge recycling method.
As shown in FIG. 2A, the conventional column driver comprises an external capacitor CEXT and a plurality of switches SW1 to SWn for switching the column lines 01 to 0n. The switches SW1 to SWn respond to a separating phase (isolation), a charge recycling phase (ext) and a driving phase (drv).
FIG. 2B shows the voltage waveforms of the column driver outputs corresponding to the separating phase, the charge recycling phase and the driving phase.
The operation of the conventional column driver adopting the 1-level/3-phase charge recycling method will be described in detail referring to FIGS. 2A and 2B.
The conventional column driver comprising the external capacitor CEXT is operated in three phases.
In the first phase, i.e., in the separating phase, the column lines 01 to 0n are separated from the output driving unit (not shown in FIG. 2A). In the second phase, i.e., in the charge recycling phase, the column lines 01 to 0n are connected to the external capacitor CEXT whereby all the charges from the column lines 01 to 0n are stored in the external capacitor CEXT; subsequently all the column lines 01 to 0n are equalized to an average voltage xe2x80x98Vdd/2xe2x80x99. In the third phase, i.e., in the driving phase, all the column lines 01 to 0n are connected to the output driving unit 14.
According to the method outlined above, the average voltage swing of the output driver may be reduced from xe2x80x98Vswingxe2x80x99 to xe2x80x98Vdd/2xe2x80x99. Thereby, it is possible to reduce power consumption of the column driver by 50% compared with the column driver which does not contain a charge recycling unit.
The 1-level/3-phase charge recycling method can be extended to a multi-level/multi-phase method to improve power efficiency, for example, a conventional column driver may adopt a 3-level/5-phase charge recycling method.
FIG. 3A shows the circuit of the conventional column driver adopting the 3-level/5-phase charge recycling method, and FIG. 3B shows the voltage waveforms of the column driver outputs corresponding to the separating phase, the charge recycling phase and the driving phase.
As shown in FIG. 3A, the conventional column driver adopting the 3-level/5-phase charge recycling method comprises three external capacitors for respective charge recycling phases.
All the column lines of the column driver adopting the 3-level/5-phase charge recycling method are not equalized to the same voltage. That is, the column lines with positive voltage are equalized to one voltage and the column lines with negative voltage are equalized to another voltage. The 3-level/5-phase charge recycling method can be extended to a higher level charge recycling method such as a 5-level/7-phase or a 7-level/9-phase, in a similar way. FIG. 3C shows the timing diagram of the column driver shown in FIG. 3A. However, conventional multi-level/multi-phase methods have the problem of timing and the necessity of the polarity reversing circuit that needs an extra control signal. The polarity reversing circuit shown in FIG. 3A is required since the charge recycled from odd lines is used to drive even lines and vice versa, as shown in the timing diagram of FIG. 3B.
It is, therefore, an object of the present invention to provide a column driving method capable of reducing the charge recycling time as well as power consumption.
It is, therefore, another object of the present invention to provide a column driving method that does not require polarity reversing circuits.
In accordance with an aspect of the present invention, there is provided a column driving method for the column driver of an LCD device comprising an LCD panel, a plurality of column lines connected to the LCD panel, an output driving unit for driving the column lines, and a charge storage unit for charge recycling the column lines, the method comprising the steps of separating the column lines from the output driving unit; transferring charges from the column lines to the charge storage unit; equalizing the voltage of the column lines; driving the column lines without using the charges stored in the charge storage unit; separating the column lines from the output driving unit; equalizing the voltage of the column lines; and driving the column lines by using the charges stored in the charge storage unit.
In accordance with another aspect of the present invention, there is provided a column driving method for the column driver of an LCD device comprising an LCD panel, a plurality of column lines connected to the LCD panel, an output driving unit having a first and second capacitor for driving the column lines, a charge storage unit having a first and second capacitor for charge recycling the column lines, and an equalizing unit connected to the column lines, the method comprising the steps of separating the column lines from the output driving unit; transferring charges from the column lines to the charge storage unit; equalizing the column lines by connecting the column lines to the equalizing unit; driving the column lines without using the charges stored in the first capacitor and second capacitor; separating the column lines from the output driving unit; equalizing the column lines by connecting the column lines to the equalizing unit; and driving the column lines using the charges stored in the first and second capacitors.
In accordance with further another aspect of the present invention, there is provided a column driving method for the column driver of an LCD device comprising an LCD panel, a plurality of odd column lines connected to the LCD panel, a plurality of even column lines connected to the LCD panel, an output driving unit for driving the column lines, a first capacitor connected to the odd column lines, a second capacitor connected to the even column lines, and a third capacitor connected to the odd column lines and the even column lines as an equalizing unit, the method comprising the steps of separating the odd column lines and the even column lines from the output driving unit; transferring charges from the odd column lines to the first capacitor and transferring charges from the even column lines to the second capacitor; connecting the odd column lines and the even column lines to the third capacitor for equalizing; driving the odd column lines and the even column lines without using the charges stored in the first capacitor and the second capacitor; separating the odd column lines and the even column lines from the output driving unit; connecting the odd column lines and the even column lines to the third capacitor for equalizing; and driving the odd column lines by using the charges stored in the first capacitor and driving the even column lines by using the charges stored in the second capacitor.